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Force Diagrams
What is force? A force is a push or a pull in a given direction. Since a force has both magnitude (the "strength" of the force) and direction, it is a vector quantity. The letter F''' is used to represent force. '''Major Players in Force Diagrams Contact Vs. Long-Range Forces * A Contact Force acts on an object only by touching it. Examples of contact forces are ::::*n'ormal forces' ::::*'frictional forces' ::::*'applied forces' * The Second type of force is Long-Range Force. A long-range force is exerted without contact. Examples of long-range forces are ::::*gravitational forces ::::*electrical f'orces' ::::*'magnetic force' Normal Forces An object at rest on a horizontal (flat) surface has no acceleration but still has weight. In this specific case the net force of the object is zero, therefore another force must exist to balance the effects of gravity. The origin of the supporting force comes from the surface. Because the force from the surface is perpendicular to both the object and itself it is called a normal force. Normal force is represented with the symbol F'''n. The Diagram on the left shows the relationship between the weight of an object and the normal force acting on it. '''Frictional Forces Frictional Forces are always present when two surfaces are in contanct. The direction of a frictional force on an object is always opposite to the direction of the object's motion. Frictional forces are also always perpendicular to and directed away from the surface. F'f is the symbol used to represent frictional force. The bright red force vector labeled Ff represents frictional force Summarizing Other Forces *Spring ('F'sp)--A restoring force exerted by a surface on an object. Spring forces are opposite the displacement of an object at the end of the spring. *Tension ('F'T)--The pull exerted by rope,or cable when attached to a body and pulled tightly. Tension force is directed away from the object and parallel to the string, rope, or cable at the point of attachment. *Thrust ('F'thrust)-- General term for the forces that move objects such as rockets, planes, cars, and people. This force points in the same direction as the acceleration of the object barring any resistive forces. *Weight ('F'g)--A long-range force dur to gravitational attraction between two objects generally Earth and an object. Weight forces point straight down toward the center of Earth. Common Misconceptions about Forces *'A force is needed to keep an object moving. If there is no net force, then the object keeps moving with unchanged velocity. If friction is a factor, then there is a net force and the object's velocity will change. *'Inertia is a force.' Inertia is the tendency of an object to resist changing its velocity. Forces are exerted on objects by the environment; they are not properties of objects. *'The quantity ''ma is a force.' The equal sign in '''F=ma' does not define ma as a force. Rather, the equal sign means the experiments have shown that two sides of the equation are equal. How Force Diagrams Show-up on the Regents! ::The correct choice is bolded * A cart is uniformly accelerating from rest. The net force acting on the cart is. (1) decreasing (2) zero (3) constant (4) increasing * When force is needed to give an electron an acceleration of 1.00 X 10^10 meters per second? (1) 9.11 X 10^-41N (2) 9.11 X 10^-31N (3) 9.11 X 10^-21N (4) 1.10 X 10^43N * Assume that an object has no unbalanced force acting on it. Which statement about the object is true? (1) The object may be in motion. (2) The object must be slowing down. (3) The object must be at rest. (4) The object may be speeding up. * If the mass of an object were doubled, its acceleration due to gravity would be (1) halved (2) doubled (3) unchanged (4) quadrupled FOR MORE REGENTS QUESTIONS ON FORCE, REVIEW CHAPTER THREE IN THE BARRON'S LET'S REVIEW: PHYSICS, THE PHYSICAL SETTING BOOK. Resources * Thrust & Weight Diagram * Physics Lecture 128: The 3rd Law * Understanding Newton's Laws * Spring Diagram * The Meaning of Force * Show Me Friction * More On Newton's Laws * Barron's Review Course Series; Let's Review: Physics, The Physical Setting * Glencoe Physics, Principles & Problems Refrences * Physics Lecture 128: The 3rd Law * Understanding Newton's Laws * The Meaning of Force * Barron's Review Course Series; Let's Review: Physics, The Physical Setting * Glencoe Physics, Principles & Problems